1. Field of the Invention
The present invention relates to a wiring substrate suited for multi-layer wiring substrate and semiconductor device package, and a method of producing the same.
2. Description of the Related Art
Multi-layer ceramic wiring substrates that allow wiring with a relatively high density have been widely used for multi-layer wiring substrates used in, for example, packaging semiconductor devices. The multi-layer ceramic wiring substrate comprises an insulating substrate made of alumina, glass ceramics or the like, and a wiring conductor made of a metal such as W, Mo, Cu and Ag coated on the surface thereof. A cavity is formed in part of the insulating substrate so that a semiconductor device is housed in the cavity. The cavity is air-tightly encapsulated with a lid.
Semiconductor devices such as IC and LSI are made with increasingly high density. Accordingly, wiring substrates used in semiconductor device packages in which semiconductor devices are mounted are required to allow for increasingly higher density, lower resistance, smaller size and lighter weight. Wiring substrates used for hybrid integrated circuits where various electronics components are mounted are also subjected to the same requirements. For these applications, a glass ceramic wiring substrate that is capable of achieving a lower dielectric constant than the alumina ceramics material and allows for lower resistance of a wiring circuit layer is viewed as promising. Glass ceramics comprises glass or a mixture of glass and a ceramic filler. The glass ceramics can be fired at a temperature lower than 1000xc2x0 C., and therefore can be fired together with a metal having a low resistance such as copper or silver, thereby making it possible to produce a wiring substrate that has a low-resistance wiring circuit layer.
When forming a wiring conductor layer on the glass ceramic wiring substrate, a metallizing paste including a wiring conductor made of a metal such as Cu or Ag as a principal component is printed by screen printing on an insulating substrate made of glass ceramics. However, with such a process, it is difficult to form wiring lines of width not greater than 100 xcexcm (particularly 80 xcexcm or less). As a result, there has been a limitation to the reduction of wiring width, posing a threat of blocking the progress toward higher packaging density, smaller size and lighter weight. There has also been such a problem that there exist many voids and grain boundaries due to the formation of the wiring conductor by means of paste; therefore, it is difficult to decrease the electrical resistance. Also the conductive paste includes a filler such as ceramics or glass added thereto, in order to alleviate the difference in thermal shrinkage between the wiring circuit layer and the insulation layer during firing. This gives rise to another cause of hampering the effort to decrease the resistance of the wiring conductor layer.
As means for solving this problem, such a technique has been known that a wiring circuit layer is formed on the glass ceramic wiring substrate by etching a metal foil (see Unexamined Patent Publication (Kokai) No. 63-14493(1998)). However, this technique has such a problem as firing the metal foil and the glass ceramics causes the substrate to warp or crack, thus making it difficult to make practical use of the technique. This is because the glass ceramics shrinks during firing although the metal foil is a dense material and hardly shrinks.
To solve these problems, such a technique is known as forming layers (restricting sheets) of an inorganic composite, that is not sintered at the sintering temperature of the wiring substrate, on both sides of the glass ceramic wiring substrate whereon the wiring circuit layer of the metal foil is formed (see Unexamined Patent Publication (Kokai) No. 7-86743(1995)). When this laminate is sintered, planar shrinkage of the wiring substrate is restricted by the inorganic composite layers, so that the metal foil and the glass ceramics can be co-fired. Also techniques for firing a green sheet coated with a conductive paste printed on the surface thereof while restricting the planar contraction by similar means are disclosed in Unexamined Patent Publication (Kokai) Nos. 4-293978(1992), 5-28867(1993) and 5-102666(1993)
However, since the wiring circuit layer is made of the dense metal foil having high rigidity, and therefore hardly experiences thermal expansion and contraction, resulting in a significant thermal stress generated in the glass ceramics. As a result, crack would occur in the surface of the glass ceramic wiring substrate having the wiring circuit layer formed thereon in the subsequent cooling process, after the co-firing, and eventually leading to the breakage of the substrate.
There has also been such a problem that the glass ceramics is prone to crack when restricting sheet layer is removed.
The glass ceramic wiring substrate of the prior art has a thermal expansion coefficient within a range from 4 to 7 ppm/xc2x0 C. at a temperature within a range from 40 to 400xc2x0 C. Meanwhile printed circuit boards comprising glass-epoxy insulation layer with a Cu wiring circuit layer formed thereon are the most frequently used for the external circuit board whereon the wiring substrate is to be mounted. The printed circuit board has a very high thermal expansion coefficient of 12 to 18 ppm/xc2x0 C. Heat generated during operation of a semiconductor device mounted on a wiring substrate or in a semiconductor device package is transmitted to both the wiring substrate and the printed circuit board. Thus as the operation of the semiconductor device is activated and stopped repeatedly, significant thermal stress is generated due to the difference in the thermal expansion coefficient between the wiring substrate and the printed circuit board.
This thermal stress acts on the periphery of the pad on the bottom surface of the wiring substrate and on the interface of joining wiring conductors of the external circuit board and terminals. This results in such a problem as the connection pad peels off the insulating substrate or the terminals peel off the wiring conductor. Thus it has been impossible to maintain stable electrical connection between the wiring substrate or the package and the printed circuit board over a long period of time.
The glass ceramics green sheet and the restricting sheet are joined by an organic component such as an organic binder included in these sheets. As a result, when the organic component has been decomposed and evaporated in the sintering process, shrinkage of the green sheet is restricted only by the frictional force between the restricting sheet and the green sheet.
However, the restricting sheet usually consists of only such components that becomes sintered at high temperature as alumina so that the restricting sheet would not be easily sintered. That is, the restricting sheet hardly includes glass component that would be turned into liquid phase, and takes the form of porous body in the sintering process.
The green sheet consists of a mixture of glass powder and ceramics filler powder. In the sintering process, the glass component contained in the green sheet forms liquid phase. The liquid phase component diffuses into the porous restricting sheet laminated on the surface of the green sheet. Consequently, glass content in the surface of the green sheet decreases and the surface whereon the restricting sheet is laminated tends to be insufficiently sintered.
Such a problem as described above is encountered more frequently as the glass ceramics composite includes more amorphous component after sintering the glass ceramics green sheet. As a result, surface of the wiring substrate becomes rough with many voids included therein. This causes such problems as lower strength of the substrate and diffusion of plating metal when the wiring circuit layer on the substrate surface is plated.
A first object of the present invention is to provide a ceramic wiring substrate that does not experience cracks or the like, and a method of producing the same.
A second object of the present invention is to provide a ceramic wiring substrate that has high reliability of connection to an external circuit board and a method of producing the same.
A third object of the present invention is to provide a ceramic wiring substrate that has good surface condition and a method of producing the same.
A fourth object of the present invention is to provide a ceramic wiring substrate that has high adhesive strength between an insulating substrate and a wiring circuit layer with high yield of production, and a method of producing the same.
The wiring substrate according to the first aspect of the present invention comprises an insulating substrate made of glass ceramics and having Young""s modulus of 120 GPa or less, and a wiring circuit layer made of a high-purity metal conductor in concentration of 99% by weight or more formed on the surface of the insulating substrate and/or inside thereof.
The wiring substrate may be a multi-layer wiring substrate that has a plurality of wiring circuit layers.
The wiring substrate can be produced by the production method comprising the steps of making a green sheet or a plurality thereof made of a glass ceramics composite that is to have Young""s modulus of 120 GPa or less after sintering, forming wiring circuit layer made of a high-purity metal conductor in concentration of 99% by weight or more formed on the surface of the green sheet or the respective surfaces of the plurality of green sheets and firing the green sheet whereon the wiring circuit layer is formed or the laminate of the plurality of green sheets whereon the wiring circuit layer is formed on each thereof at a temperature below the melting point of the high-purity metal conductor.
According to the present invention, since resistance of the wiring circuit layer is decreased by the use of the high-purity metal conductor and the insulating substrate has a low value of Young""s modulus, cracks can be prevented from occurring. Such a wiring substrate can be provided that has excellent reliability of connection when mounted on a printed circuit board.
The wiring substrate according to the second aspect of the present invention comprises an insulating substrate made of glass ceramics, and a wiring circuit layer that is formed at least in the inside of the insulating substrate and is made of a metal conductor in concentration of 99% by weight or more, wherein a difference in the thermal expansion coefficient between the insulating substrate and the wiring circuit layer is 14 ppm/xc2x0 C. or less at a temperature within a range from 25 to 800xc2x0 C.
The wiring substrate according to the second aspect of the present invention can be produced in the production method comprising the steps of forming one or a plurality of green sheets made of glass ceramics which is to have thermal expansion coefficient that is different from that of the wiring circuit layer by 14 ppm/xc2x0 C. or less at a temperature within a range from 25 to 800xc2x0 C. after sintering, forming the wiring circuit layer made of a high-purity metal conductor in concentration of 99% by weight or more on the surface of the green sheet or the surfaces of the plurality of green sheets and firing the green sheet whereon the wiring circuit layer is formed or the laminate of the plurality of green sheets whereon the wiring circuit layer is formed on each thereof at a temperature below the melting point of the high-purity metal conductor.
According to the present invention, since the insulating substrate and the-wiring circuit layer have a small difference in the thermal expansion coefficient therebetween, cracks can be suppressed from occurring during cooling after the sintering process. Also it is made possible to provide the wiring substrate having high reliability that is capable of maintaining stable electrical connection with an external circuit board made of such a material as glass epoxy over a long period of time.
The wiring substrate according to the third aspect of the present invention comprises an insulating substrate that is made of glass ceramics and includes 20% by volume or more amorphous component with less than 5% of the outer-most surface thereof being occupied by voids, and a wiring circuit layer made of a high-purity metal conductor of 99.5% by weight or more content formed on the surface of the insulating substrate and/or inside thereof.
The wiring substrate of this configuration can be produced in the production method comprising the steps of forming one or a plurality of green sheets made of glass ceramics composite that includes 20% by volume or more amorphous component after sintering, forming the wiring circuit layer made of a high-purity metal having conductor of 99.5% by weight or more content on the surface of the green sheet or the surfaces of the plurality of green sheets, forming a laminate having a restricting sheet that includes a low-sinterable ceramic material as a principal component and an amorphous component in 0.5 to 15% by volume being laminated on the surface on at least one side of the one green sheet or the green sheet laminate of the plurality of green sheets, firing the laminate at a temperature below the melting point of the high-purity metal conductor thereby to make the glass ceramic wiring substrate having the restricting sheet formed on the surface thereof, and removing the restricting sheet from the glass ceramic wiring substrate.
According to the present invention, since such a ceramic wiring substrate can be obtained that is provided with the wiring circuit layer made of a low-resistance metal such as Cu and Ag, wherein planar shrinkage can be restricted, finer wiring can be made thereon and the substrate surface is in good condition.
The wiring substrate according to the fourth aspect of the present invention comprises an insulating substrate made of ceramics and a wiring circuit layer made of a high-purity metal conductor of 99% by weight or more embedded in the ceramic insulating substrate so that the surfaces thereof are flush, wherein the wiring circuit layer has a cross section of inverted trapezoid shape in the direction perpendicular to the wiring direction. The angle made between the bottom and sloped side of the inverted trapezoid shape is preferably within a range from 45 to 80xc2x0.
The wiring substrate of this configuration can be produced in the production method comprising the steps of forming one or a plurality of green sheets made of ceramic insulating material, forming the wiring circuit layer having trapezoidal cross section in the direction perpendicular to the wiring direction by etching the high-purity metal conductor layer including 99% by weight or more metal component bonded onto the surface of a transfer film, transferring the wiring circuit layer by pressing the transfer film having the wiring circuit layer formed thereon onto the surface of the green sheet, embedding the wiring circuit layer into the surface of the green sheet and then peeling off the transfer film, and firing the one green sheet whereon the wiring circuit layer is formed or the laminate of the plurality of green sheets whereon the wiring circuit layer is formed on each thereof at a temperature below the melting point of the high-purity metal conductor.
According to the present invention, the ceramic wiring substrate having the wiring circuit layer of good adhesion with the insulating substrate can be obtained.
The above objects and other objects, features and effects of the present invention will be made apparent from the description of preferred embodiments that follows with reference to the accompanying drawings.